Christ mythicists like Richard Carrier deny the historicity of Jesus. In addition, Bart Ehrman denies the reliability of testimonial evidence. Let's briefly touch on some elementary problems with that stance:
1. It's a double-edged sword. If you deny that even firsthand sources are generally reliable, then secondhand sources will be even more unreliable. Yet critics of the Bible rely on secondhand sources to impugn the historicity of Scripture.
i) For instance, they say Daniel mispredicted the death of Antiochus Epiphanes (c. 215-164 BC). That, however, depends on how much stock you put in extrabiblical sources, viz. Appian (2C AD), Diodorus Siculus (1C BC), Polybius (1 BC), 1 Maccabees (c. 90 BC), 2 Maccabees (c. 78-63 BC).
Most of these weren't even by contemporaries of Antiochus IV. None of them were eyewitnesses. Given Ehrman's historical skepticism, why use sources like that as benchmarks to judge the historical accuracy of Daniel?
That's even assuming the oracle in question refers to Antiochus IV rather than a future Antichrist.
ii) Or take Qurinius (c. 51 BC–c. 21 AD). Critics say Luke's reference to his "census" is a historical blunder.
Now, there are literary notices regarding Quirinius in Dio Cassius (c. 164–235 AD), Florus (2C AD), Josephus (c.37–c. 100 AD), Suetonius (c. 71–c. 135 AD), and Tacitus (c. 56–c. 120 AD).
None of these writers were contemporaries of Quirinius. Not even younger contemporaries. All of them were writing generations after the fact. Given Ehrman's historical skepticism, why uses sources like that as a benchmark to judge the historical accuracy of Luke?
2. Consider a more radical stance. You have omens, portents, prodigies, miracles, and apotheosis attributed to Roman emperors like Julius Caesar, Augustus, Vespasian, and Trajan. Ancient accounts of Roman emperors can check numerous boxes in the mythotypes of Lord Raglan and Joseph Campbell. Given the legendary embellishment of Roman emperors by Greco-Roman historians, if we apply mythicist principles in their case, we ought to conclude that Julius Caesar, Augustus, Vespasian, and Trajan never existed! These are fictional characters who exhibit the same mythical traits as Romulus, Remus, Perseus, Theseus, Hercules, Achilles, Aeneas, &c.